Private Branch Exchanges (PBXs) are typically used by a corporation to provide voice/data (Voice/Facsimile/Modem/Data) between various facilities within the corporation. Referring now to FIG. 1, there is shown a prior art network 10 comprising a plurality of PBXs 12, 14, and 16 which are interconnected with communication lines 17, 18, and 19. More particularly, PBX 12 is interconnected to PBX 14 and PBX 16 via communication lines 17 and 18, respectively, while PBX 14 is interconnected to PBX 16 via the communication line 19. Therefore, each PBX 12, 14, and 16 can directly communicate with any of the other PBXs in the network 10. If two PBXs need to communicate, then a separate connection between the PBXs is required. This configuration becomes increasingly more inefficient as the number of PBXs increases because the number of leased communication lines required becomes excessive in order to keep connectivity between the PBXs.
Referring now to FIG. 2, there is shown a prior art network 20 which reduces the number of leased communication lines needed in a large network and illustrates the known concept of a "Transit PBX". The network 20 comprises a plurality of remote PBXs 22, 24, and 26, and a remote Transit PBX 28 that is interconnected to the remote PBXs 22, 24, and 26 via separate leased communication lines 25, 27, and 29, respectively. For example, the PBXs 22, 24, and 26 could be located in Los Angles, New York, and Dallas, respectively, with the Transit PBX 28 located in Chicago. All call control between the PBXs 22, 24, and 26 is performed within the single box of the Transit PBX 28.
Referring now to FIG. 3, there is shown a prior art network 20a which includes four Transit PBXs similar to the Transit PBX 28 of FIG. 2 that are interconnected. The network 20a comprises a plurality of Transit PBXs of which four Transit PBXs 28a, 28b, 28c, and 28d are shown. The Transit PBX 28a is interconnected to a remote PBX 22a via a leased communication lines 25a, and is interconnected to the Transit PBX 28b via a link 23a. The Transit PBX 28b is also interconnected to Transit PBXs 28c and 28d via separate leased communication lines 23b and 23c, respectively. The Transit PBX 28c is interconnected to PBXs 22b and 22c via separate leased communication lines 25b and 25c, respectively. The Transit PBX 28d is interconnected to PBX 22d via a leased communication lines 25d. All call control between the PBXs 22a, 22b, 22c, and 22d is performed within the single box of each of the Transit PBX 28a, 28c, or 28d interconnected to the associated PBXs. Still further, each of the Transit PBXs 28a, 28b, and 28c need to be manually updated to include any changed route information occurring in each of the other Transit PBXs.
U.S. Pat. No. 4,488,004 (Bogart et al.), issued on Dec. 11, 1984, discloses an arrangement for according station and attendant feature transparency between independently operational Private Branch Exchanges (PBXs) of a PBX network. Each PBX includes a switching processor, station lines, and tie trunks extending to other PBXs, and also includes a Direct Memory Data Controller (DMDC) for transmitting service or feature related data messages associated with tie-trunk calls to other PBXs over data links. On an interPBX call, the DMDC in the calling PBX transmits a data message to a DMDC in the called PBX over a direct link between the calling and called PBXs. In the event there is no direct link between the calling and called PBXs, the DMDC in the calling PBX transmits a message over a data link to a DMDC at a tandem PBX which, in turn, sends the message over a link to the DMDC in the called PBX. The DMDC in the called PBX processes the received data message and, by direct memory access, sends the message to its switch processor which provides the specified feature or other service required on the call above and beyond the establishment of a voice path connection between the calling and called PBXs. Such arrangement is similar to the network of FIG. 1.
U.S. Pat. No. 5,422,943 (Cooney et al.), issued on Jun. 6, 1995, discloses a Private Branch Exchange (PBX) network comprising at least two PBXs where each PBX serves a group of stations of a customer, and at least two Serving Switching Systems (SSSs) for serving the PBXs which are interconnected via a public switched telephone network. The PBXs are each connected to one of the SSSs via at least one Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI) facility. The SSSs are interconnected for transmitting ISDN signals, and the ISDN stations on one of the PBX can communicate, using ISDN signals, with ISDN stations on another one of the PBXs via ones of the SSSs. Call control signals for interPBX calls of the customers of the PBX network are exchanged between a specific calling PBX and a SSS to which the specific PBX is connected. Ones of the SSSs comprise translation data for associating any internal customer directory number with any customer station.
U.S. Pat. No. 5,212,691 (Hokari), issued on May 18, 1993, discloses a private network where Private Branch Exchanges (PBXs) are connected directly through a dedicated tie trunk to allow direct interworking between the PBXs, and through a Primary Rate Interface (PRI) to an Integrated Services Digital Network (ISDN). One of the PBXs (a master PBX) has (a) routing data for virtual tie trunks for signaling and speech transmission to be established via the ISDN to a distant PBX, and (b) channel status data indicating their set-up or clear-down state according to time of day and calender day. The stored data is periodically accessed and, depending on the contents of the channel status data, a call set-up or clear-down message is sent from the master PBX to the ISDN to set-up or clear-down the virtual tie trunks. On receipt of a response from the distant PBX, busy/idle status of the virtual tie trunks is changed according to the accessed channel status data. In response to a request from a user station, a path is established therefrom to the virtual speech tie trunk and a call set-up message is sent directly to the distant PBX through the virtual signaling tie trunk.
The prior art references teach that the PBXs of a network act as separate PBXs or nodes that handle call control for that PBX or node. It is desirable to provide an arrangement wherein call control is distributed across a wide area network instead of solely within a single PBX or node.